The invention relates to surface-improving additives for cathodic electrodeposition coating (German abbreviation KTL), and relates more particularly to addition of anti-pitting agents.
Cathodic electrodeposition coating (KTL) is a frequently used method, particularly for priming car bodies, in which water-thinnable synthetic resins bearing cationic groups are deposited on to electrically conductive members, using dc. This method is described in the literature and widely used in practice. In it, a workpiece having an electrically conductive surface of metal or electrically conductive plastics is placed in an aqueous electrodeposition bath and connected as cathode to a dc source, after which the lacquer on the surface is coagulated by the current (EP-B 0 066 859, EP-A-0 004 090, DE-A-32 15 891). The electrodeposition bath comprises an aqueous dispersion, e.g. a suspension or emulsion or aqueous solution, of one or more binders made water-dispersible by partial salt formation with organic or inorganic neutralising agents, and of pigments, fillers and additives dispersed therein, dispersed solvents and other adjuvants.
Lacquer materials having the aforementioned composition are designed to produce smooth surfaces after deposition and cross-linking of the resulting film. The surfaces uniformly cover faults in the substrate. They are used to produce a substrate for the subsequent layers, so as to obtain a high-quality coating lacquer. In practice, however, surface faults, particularly pits, frequently occur in the KTL film after stoving, or the surfaces are non-uniform. These faults may be due to the electrodeposition coating materials used, but frequently originate from impurities which find their way into the electrodeposition coating bath. The impurities from the lacquer material may be gel particles from binder manufacture, damaged resin particles from pigment paste manufacture, pigment impurities or foreign substances from equipment for producing the corresponding materials. The subsequently-introduced foreign substances may e.g. be deep-drawing grease, anti-corrosion grease, seam-sealing materials or substances used in pretreatment.
Another class of impurities are deposited from the air after the electrodeposition lacquer coating has been deposited and before the lacquer film has become cross-linked. Examples of such impurities are fluorine-containing or silicone-containing aerosols or lubricants from the systems required for conveying the parts for coating.
These substances may enter the lacquer film before stoving and produce surface faults such as pits during stoving e.g. because of incompatibility. The damage caused by these materials cannot be predicted but must be determined by experiment.
These surface faults necessitate expensive after-treatment, to obtain a smooth surface for the subsequent layers. To ensure continuous production, surface faults of this kind must be avoided. It is difficult to eliminate the varied causes of pits, and therefore the usual aim is to avoid pitting by using additives.
Silicone oils or fluorine-containing organic substances are known examples of anti-pitting additives. These eliminate pits in the layer in which they are used, but often cause serious surface faults in subsequent layers. Also, the subsequent layers do not adhere firmly to the treated substrates. These additives are therefore unsuitable for KTL layers.
An attempt has therefore been made to find other non silicone-containing additives, in order to avoid these frequent surface faults. Polyoxyalkylene polyamine reaction products with epoxy resins are additives to KTL systems and are described in US-A-4 810 535. The aim is to avoid pits. EP-A-0 324 961 describes polyoxyalkylene polyamines which are added to KTL baths in order to obtain fault-free film surfaces. DE-A-38 30 626 describes modified acrylates which are additives to KTL baths and are optionally neutralised before being added to the lacquer material. The aim as before is to avoid various kinds of surface faults. EP-A-0 301 293 describes cathodic electrodeposition coating baths in which the additives are homopolymers or copolymers of an alkyl vinyl ether. All these materials have to be added in relatively large quantities to have an anti-pitting effect. This results in a difficulty, in that there is a marked adverse effect on adhesion to subsequent layers, e.g. fillers or polyvinyl chloride materials.
Irrespective of the cause of pits, the efficiency of the known anti-pitting agents for KTL systems is inadequate in practice. Even large amounts of these additives do not completely eliminate pits, and consequently the substrates frequently require after-treatment, e.g. by grinding or additional lacquering.